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PCBD-Oct2017

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26 The PCB Design Magazine • October 2017
As Dave Dunham of Molex Corp. likes to
say, "When designing high-speed serial links
beyond 10 GB/s, everything matters." And part
of that everything is accurate modeling of trans-
mission line losses.
Failure to account for conductor roughness
can ruin you day especially if you are trying to
push 28 GBaud/s (56 GB/s) PAM-4 signaling
down your channel. As shown in Figure 1, with
just 1.4 dB delta in insertion loss, at 14 GHz Ny-
quist frequency, results in an average reduction
of 38% in eye height, and 4.3% increased jitter
across all three eyes, when conductor roughness
is taken into account. On top of that, failure to
correct dielectric constant (D
k
) from manufac-
turers' data sheets due to conductor roughness
can lead to inaccuracy in phase delay
[1]
.
To ensure first time success at these speeds,
using the right parameters for dielectric and
conductor roughness to feed into modern EDA
tools is a prerequisite. This is especially true for
long backplane channels.
Many EDA tools include the latest and great-
est models for conductor surface roughness and
wide-band dielectric properties. But obtaining
the right parameters to feed the models is al-
ways a challenge. So how do we get these pa-
rameters?
One way is to follow the design feedback
method which involves designing, building
and measuring a test coupon. After model-
ing and tuning various parameters to best fit
measured data, D
k
, D
f
and roughness param-
eters can be extracted. They are then used in
by Bert Simonovich
LAMSIM ENTERPRISES
Practical Modeling of
High-Speed Backplane Channels
FEATURE COLUMN: BERT'S PRACTICAL DESIGN NOTES
Figure 1: Simulated results of a differential transmission line with and without conductor roughness taken
into account. With just 1.4 dB delta in insertion loss, as shown on the left, results in an average reduction
of 38% in eye height and increase of 4.3% in jitter across all three eyes, as shown on the right.